Wireless Charging (also known as inductive charging or cordless charging) is a type of wireless power transfer. It uses electromagnetic induction to provide electricity to portable devices. The most common application is the Qi wireless charging standard for smartphones, smartwatches and tablets. Inductive charging is also used in vehicles, power tools, electric toothbrushes and medical devices. The portable equipment can be placed near a charging station or inductive pad without needing to be precisely aligned or make electrical contact with a dock or plug.
Energy is transferred through inductive coupling. An alternating current is run through an induction coil in the charging station or pad (the primary or transmission coil.) Any moving electric charge creates a magnetic field, as stated by Oersted's law. The magnetic field fluctuates in strength as the AC current is continually changing amplitude. A changing magnetic field generates an electromotive force otherwise known as Faraday's law of induction. This makes an alternating electric current in a second induction coil (the receiving, or secondary coil) in the portable device. It is then converted to direct current with a rectifier and used to charge a battery or provide operating power.
Greater distances between sender and receiver coils can be achieved when the inductive charging system uses resonant inductive coupling, where a capacitor is added to each induction coil to create two LC circuits with a specific resonance frequency. The frequency of the alternating current is matched with the resonance frequency, and the frequency chosen depending on the distance desired for peak efficiency. Recent improvements to this resonant system include using a movable transmission coil (i.e., mounted on an elevating platform or arm) and the use of other materials for the receiver coil such as silver-plated copper or sometimes aluminum to minimize weight and decrease resistance due to the skin effect.